1. Field of the Invention
This invention relates to a surface treatment method for aluminum or aluminum alloys and more particularly, to a surface treatment method for aluminum or aluminum alloys, which is effective for pretreatment in case where UBM (under bump metal) or bumps are formed on a wafer by plating.
2. Prior Art
For the formation of an UBM or bump on a silicon wafer, the usual practice is to use a method wherein an aluminum thin film electrode in the form of a pattern is subjected to zinc substitution treatment to form a substituted zinc film on a wafer, followed by forming bumps by electroless nickel plating, a method wherein palladium treatment is performed in place of the above zinc substitution treatment, after which bumps are formed by electroless nickel plating, or a method wherein an aluminum thin film electrode is substituted on the surface thereof directly with nickel and bumps are formed by self-catalytic electroless nickel plating.
In the formation of UBM or bumps by use of any of such methods as mentioned above, it is usual to perform, as a pre-treatment stage, a defatting treatment against the aluminum thin film electrode, a treatment of removing an aluminum oxide film or metal impurities from the aluminum thin film electrode, and the like. In this case, with an aluminum oxide film, taken out among similar aluminum oxide films, whose thickness is very thin as will be caused by nitric acid immersion, plating treatment is possible without inviting any problem if the plating treatment is performed as it is in a subsequent step. Nevertheless, where such a firm aluminum oxide film as is formed in manufacturing steps including a machining or cutting step and an annealing step is left on the surface, adhesion of a plated layer formed in a subsequent step may become inadequate or holes may be formed in the plated layer in some cases, or no plated layer may be deposited in the worst case. Accordingly, such a firm aluminum oxide film has to be completely removed beforehand.
In order to cope with the problems set out above, there has been proposed a method (see JP-A 11-87392) wherein a plated underlying layer is formed by a dry process without dissolution of an aluminum oxide film. However, this method still has rooms for improvements in view of the facts that the procedure of the method is complicated, the method is disadvantageous in promptness and production costs, and because the residual oxide film is insulating in nature, a thermal resistance increases with the result that electric characteristics become worsened.
It will be noted that mention is made, as prior art references, of JP-A 2004-346405 and JP-A 2007-254866, aside from the above-mentioned Laid-open Patent Application.